Field of the Invention
This invention relates to a process for producing colour change devices incorporating areas of contrasting colour and to the devices thus produced.
II. Description of the Prior Art
Colour change devices are articles which exhibit a particular colour in their normal state but undergo a visible change (or loss) of the original colour when physically disturbed in some way, such as being pulled apart, punctured, bent or cut. Items of this kind are frequently used as tamper evident devices for protecting containers or packages since the visible change of colour provides an indication of tampering if the devices are used in such a way that they have to be physically disturbed for the package to be opened.
Our prior U.S. Pat. No. 4,837,061 issued on June 6, 1989 (the disclosure of which is incorporated herein by reference) relates to a basic procedure for producing colour change devices of this kind which rely on the generation of a visible colour by a light interference and absorption effect created by optically thin anodic films grown on certain metals (e.g. valve metals such as Ta and Nb, etc.). In devices of this kind, an irreversible colour change is produced when the anodic film is detached from the underlying metal and this detachment is made possible by conducting the anodization required for the growth of the film on the metal in an electrolyte containing an adhesion reducing agent, such as a fluoride. If the anodization is carried out without an adhesion reducing agent of this kind, the resulting anodic films adhere tenaciously to the underlying metal, making the required change of colour impossible to bring about.
Our patent also mentions procedures for incorporating so-called "latent indicia" into the colour change devices. Latent indicia are patterns, messages, codes, ornaments, or the like, which are invisible prior to the colour change but which become visible after the colour change has taken place. In this way, it is possible to include warnings, such as "package opened", into the devices to provide further protection. The procedures disclosed for accomplishing this require a two step anodization treatment in which the effect of the adhesion reducing agent is first masked from some areas of the metal during anodization but not from others, and then normal anodization is carried out. As a result, the anodic film is detachable from some areas of the device (resulting in a colour change) but not from others (resulting in no colour change). The resulting areas of contrasting colour form a visible pattern or message.
Our copending U.S. patent application Ser. No. 07/510,175 filed on Apr. 17, 1990 (the disclosure of which is incorporated herein by reference) relates to an improved way in which latent indicia can be introduced into colour change devices of the kind described in our prior patent. According to the improved procedure, a single anodization step can be carried out in an electrolyte containing the adhesion reducing agent after first creating the desired pattern or message on the metal using a mask which blocks the effect of the adhesion reducing agent in the areas immediately beneath the mask without otherwise affecting the formation of the anodic film beneath the mask. This results in the formation of the desired detachable and non-detachable areas of the anodic film required to produce the areas of contrasting colour when the device is activated (i.e. when the anodic film is detached).
Our further copending patent application Ser. No. 07/540,937 filed on June 20, 1990 (the disclosure of which is also incorporated herein by reference) describes a further refinement in which flexible colour change devices can be made to undergo activation (colour change) by bending rather than delamination.
While our prior patent and copending applications produce colour change devices which are extremely useful, particularly as tamper evident structures, they do not disclose ways of producing colour change devices which initially have areas of different colour that take on the same colour after activation, i.e. devices which incorporate "disappearing indicia" rather than latent indicia. It has now been found that devices of this kind have important commercial uses. For example, product identification codes are computer readable bar designs applied to almost all articles sold these days but their use creates certain disadvantages. If the codes are printed on labels applied to the articles to be sold, the labels may be fraudulently removed and adhered to other articles, e.g. articles of higher cost, before the codes are read at the point of sale. On the other hand, if the codes are printed directly onto the article to be sold or its packaging, the codes cannot be removed and remain permanently visible as unattractive markings on the article or packaging. While this latter disadvantage may not be of much concern in the case of everyday items, it is a serious disadvantage for certain high cost, decorative or fashion items, such as expensive perfumes and the like which are sold in appealing containers or packages. In such cases, it would be advantageous to make the bar codes easily removable yet non-transferable.
While colour change devices incorporating disappearing indicia offer a way of achieving this goal, as indicated above our U.S. patent and copending applications provide no method of producing such devices.